Download Patterns of Biodiversity I

Community Ecology
I. Introduction
II. Multi-species Interactions within a Trophic Level
III. Multi-species interactions across Trophic Levels
IV. Succession
A. Definitions
B. Types of Succession
C. Mechanisms
1. Facilitation
Pioneer species change the environment in a way that INCREASES the ability of other species to
colonize. Indeed, later species can not colonize unless the pioneers have done so. Bare rock
must be colonized by lichens and moss before enough soil is created to support vascular plants.
Carrion beetles must open holes in a carcass before fly maggots can gain access. Sphagnum
moss must extend over a bog before enough windblown soil and decaying detritus builds up to
support vascular plants.
2. Tolerance
Here, late species tolerate the presence of early species but don’t ‘need’ them.
Many secondary successional sequences seem to behave this way, where early
species get in sooner and dominate, but don’t really affect the ability of later
species (like trees) to colonize… in fact, they may all colonize at the same time,
but since trees grow more slowly, the weeds dominatefor the first few years.
3. Inhibition
Early species inhibit the success of later colonists. This creates different seres, depending
on who gets their first… this is called a ‘priority effect’, and it means that the structure of
the community is dependent on the order of colonization. If bryozoans colonize reef
substrate, tunicates and sponges can’t.
D. Model (Tilman 1985)
Succession can be modeled by Tilman competition models, if the relative abundance of critical nutrients is
inversely correlated. Then, we may get a sequence of predictable species colonizing, coexisting, and being
outcompeted as one resource declines while another increases.
E. Community Patterns
- The characteristics of early and late successional communities change, from pioneer
communities with open nutrient cycling, little competition, small generalist r-selected
species in low diversity, linearly connected webs to late successional communities with
closed nutrient cycles in competitive environments dominated by a lot of biomass of large
specialized K-selected species linked in complex, highly connected complex webs.
V. Biodiversity: Patterns and Processes
A. The Species-Area Relationship
1. The Pattern
Almost all organisms – across different taxonomic scales and physiologies, show this
pattern. As sampling area increases, species diversity increases. Why?
2. The Theory of Island Biogeography
MacArthur and Wilson (1967) suggested that species diversity in a community might
be an equilibrium between colonization (adds species) and extinction (subtracts
species). The created a model which considered how these effects might play out on
“islands” of isolated habitat, being populated from a source ‘mainland’ with a
particular fixed species pool.
a. Islands Size Effects
- The colonization rate to large islands should be higher than the colonization rate to
small islands… as a combination of large islands being larger ‘targets’, and as having
more habitats so that the colonists would have a better chance of establishing a
population. This has been confirmed by sampling experiments, except among small
islands (differing in size) which don’t have very different habitats.
- The extinction rate, however, should be higher on small islands. Small islands will
have fewer resources and will house smaller populations which are more likely to bounce to zero randomly.
- Now, when we represent these functions in relation to species richness, things work like this: The colonization
rate is a decreasing function of species richness… because as species richness increases on an island, the
probability GOES DOWN that the next colonist from the mainland species pool will be a NEW one to the island.
However, the extinction rate is an increasing function of diversity, because the more species there are, the more
likely it is that one of them, by chance, will go extinct.
- So, if we combine the decreasing colonization functions and the increasing extinction functions for large and
small islands, we see that the functions intercept at a higher richness value on large islands than on small
islands… meaning that large islands have a higher EQUILIBRIUM species richness value than small islands.
Species are being added and lost all the time, but the equilibrium richness is maintained and is higher on large
islands.
b. Island Isolation Effects (Distance from Source Populations)
- MacArthur and Wilson realized that isolation could have similar effects, in that more distant islands would
have lower colonization rates, and higher extinction rates, than close islands. The extinction rates would be
higher because, once extinction occurs, a distant island would be far less likely to be recolonized again. In a
sense, near islands might even maintain themselves just by migration – continually ‘rescued’ from extinction by
migrants.
c. Empirical Tests
- Porcasi et al. (1999) – Catalina islands off California. Over 50 years, the number of
bird species is fairly constant, yet there has been a significant change in the actual
membership of these communities (turnover).
- Simberloff and Wilson (1970) manipulated mangrove islands in the Florida Keys…
collecting all the isects by fumigation, and then looking at the pattern of recolonization.
Islands tended to recover to richness values near the original levels, and yet there was continual turnover.
3. Why is this important? Fragmentation
This theory is one of the most important contributions to community ecology, because
it is so easily genralizable to a variety of habitats at many scales. In fact, one might
even consider continents to be islands with barriers to dispersal between them.
Habtats are naturally subdivided – be they separated by rivers, land, or elevational
change.
And of course, one of the most important effects that humans have had on natural landscapes is to reduce their
size and subdivide them. So, simply as a consequence of the reduction in size of an area, we should expect local
species extinctions and a reduction in species richness.
Study Questions:
1. Distinguish between the facilitative, tolerance, and inhibitory models of succession, and give an example of
each.
2. List six characteristics of early and late successional communities, and 4 characteristics of the life-history of the
species that dominate these communities.
3. Give two reasons why colonization rate should increase with island area, and one reason why extinction rate
should decline with islands area.
4. Describe the results of the surveys in the catalina islands and the mangrove islands in the keys. What essential
features of these studies provide support for the equilibrium theory of species richness?
5. Why is the theory about “islands” become so important to issue of conserving biodiversity?